Interline transfer CCD imagers have, separate from the CCD portions of the imager, lines of photosensors in which elements (or pixels) of a radiant energy image are photoconverted to charge packets. These CCD imagers use a parallel array of CCD line registers (i.e., a field register) shielded from or not photosensitive to image irradiation, for transporting lines of charge packets representative of image element samples. These charge packets are transferred into the CCD line registers from the photosensors at selected times. These charge packets are transported to load in parallel the successive charge transfer stages of a CCD output line register, also shielded from or not photosensitive to image irradiation. The output line register is serially unloaded to supply charge packets in raster-scan order to a charge sensing stage for conversion to video signal voltage samples.
The charge packets descriptive of image elements are generated by parallel lines of photosensors adjacent to or stacked on top of the CCD registers, and these charge packets are transferred into charge transfer stages in the CCD register during retract intervals, each stage being respectively connected to a particular photosensor. The photosensors in some interline transfer imagers are provided by photocharge collection regions induced under gate electrodes and in portions of the substrate of semiconductive material, other than those portions taken up by CCD registers. The photosensors in other interline transfer imagers are provided by photodiodes which may be disposed in or over the substrate to provide photocharge collecting regions. Visible light imagers constructed on a silicon substrate normally have photosensors comprising photocharge collecting regions disposed in the substrate. The photocharge collecting regions may be made by selectively doping the bulk semiconductive material to form photodiodes, or they may be induced in the bulk semiconductive material by applying appropriate voltages to overlying gate electrode structures. Infrared imagers constructed on a silicon substrate use photosensors disposed over the substrate; the photosensors may be platinum silicide/platinum Schottky barrier diodes, for example.
The parallelly arrayed CCD registers may be "column" registers disposed perpendicularly with respect to the lines of photosensors, with transfer of fields of charge packets being made during field retrace intervals. Or, in the case of charge sweep devices (CSD) imagers, the charge packets may be transferred into the column registers a line at a time during each line retrace interval in the field trace interval and transferred through the column registers during the following line trace intervals, to be integrated and transferred into the output line register during line retrace intervals. Alternatively, the parallelly arrayed CCD registers may be "row" registers disposed parallelly with respect to the lines of photosensors, with transfers of lines of charge packets being made during line retrace intervals.
Where there is need for shielding CCD shift registers from irradiation by the image, it has been prior art practice to project the radiant energy into photosensors located on the same side of the substrate that the gate electrodes of the CCD registers are disposed on. That is, "front illumination" has been employed, so the irradiation shields can be emplaced in accurate registration with the CCD registers themselves, using additional photolithographic steps.
"Fill factor" in CCD imagery is a term used to describe what portion of the radiant energy in an image is made available to the photosensors, and a fill factor approaching 100% is desirable from the standpoint of imager sensitivity. The fill factors associated with the front-illuminated interline transfer imagers are low compared to those of other types of imager, such as the field transfer CCD imager, which are constructed on thinned semiconductor substrates and are "back-illuminated". (The terms "front-illumination" and "back-illumination" are used by television camera designers, particularly designers of visible-light-sensitive television cameras. The "front" and "back" correspond to the "top" surface and "bottom" surface in the convention that solid-state device makers customarily use to describe the imager die). The interline transfer imagers tend to have relatively low fill factors because there is no photoconversion of the portions of the image falling on the radiation shields over the CCD registers. Some attempts have been made in the prior art to improve fill factor for interline transfer imagers using lenticular lens structures to concentrate image energy on the lines of photosensors, diverting image energy away from the radiation shields over the CCD register, where it would be reflected or absorbed and thermally dissipated.
Insofar as improving fill factor is concerned, back-illumination of a CCD imager of interline transfer type is not advantageous as long as radiation shields are used with the CCD registers. The thinned semiconductor substrate used for a back-illuminated imager is notorious for its tendency to buckle along its expanse when held down to a planar surface (e.g., by a vacuum hold-down), so it is difficult to obtain registration between the photolithographic processes used to define the irradiation shields and the CCD registers when they are disposed on opposite surfaces of the substrate. The irradiation shields on the opposite surface of the substrate from the CCD registers are further away from these CCD registers than if they were on the same surface of the substrate. So, undesirably, there is a greater tendency for irradiation to enter each CCD register at an angle under the edge of its irradiation shield. To cope with these problems, the irradiation shields have to be wider on a back-illuminated surface than on a front-illuminated one, which tends to reduce fill factor.
Even with irradiation shields disposed over the CCD registers of an interline transfer imager, irrespective of whether front- or back-illumination is employed, there is a tendency for them to accumulate some photocharge. This attributable to diffusion of charge carriers through the substrate from portions of the substrate bulk, where photoconversion takes place, under the channel stops defining the charge transfer channels of the CCD registers and up into the potential wells induced in the CCD registers at or near the imager top surface. This tendency has been forestalled in front-illuminated interline transfer imagers manufactured by Sharp and by Toshiba by placing buried implantations of a deep dopant (e.g., ion-implanted boron in a p- type silicon substrate), so that carriers from the bulk underlying the charge integrity sites cannot diffuse into the CCD registers through their bottoms. These buried implantations are similar to the buried implantations commonly used under anti-blooming drains and are placed by the same manufacturing steps.